This invention relates to novel coating compositions and more specifically to corrosion resisting coatings which can be applied directly on to various surfaces particularly metal and polymeric composite substrates as a single topcoat.
Various surfaces and particularly metal substrates require the protection of a coating especially when the surfaces are exposed to a corrosive environment. Metal surfaces on aircraft for example are exposed to seawater spray which require protection from corrosion due to salt, etc. Specifically, aircraft, i.e., Navy aircraft, which are exposed to aircraft carrier environment are subjected to seawater spray in addition to various acid forming gases such as sulfur dioxide and carbon dioxide. Moreover, in addition to aircraft, various machinery and equipment in the industrial environments where fossil fuels are used need protection against corrosion. In addition to corrosion, it is important that the coatings be resistant to various chemicals, the weather, be flexible and have good adhesion.
Presently, coating systems comprise one or more films, i.e., an undercoat and a topcoat. Aircraft have been traditionally coated with high performance two-component protective coatings generally consisting of an epoxy primer and a polyurethane topcoat. The epoxy primers used on the aircraft are designed to adhere to the metal surface and improve the adhesion of the topcoat and prevent corrosion. However, the primer coat requires a topcoat, since it lacks flexibility especially at low temperatures (-60.degree. F.) which results in extensive cracking in highly flexed areas of the aircraft. The primer also lacks weather resistance and cannot generally be formulated in various colors required for aircraft. Thus, the polyurethane compositions of this invention provides not only resistance to the weather and various chemicals, i.e., NaCl, SO.sub.2 and CO.sub.2, but also have the required degree of flexibility and the desired optical properties. Moreover, the multi-film coating systems utilized heretofore generally have a total dry film thickness ranging up to about 0.005 inches, e.g., up to about 5 mils or more which adds considerable weight to the aircraft. Further, it is very time consuming to apply two coats particularly since there is a drying time requirement between each application. The removal of a two-coat system also can be difficult and time consuming and the prior coatings generate high levels of volatile organic component (VOC) emissions during the coating operations.
In accordance with this invention, however, the corrosion resistant coating comprises a polyurethane binder derived from the reaction of at least one polyester polyol and a diisocyanate in combination with a unique pigment system consisting essentially of an alkaline earth phosphate particularly zinc phosphate or a zinc-barium phosphate, a zinc salt of benzoic acid or substituted benzoic acids, and zinc molybdate. All three of the zinc salts are critical in the relative proportions to provide a high gloss film with the necessary corrosion resistance required of a coating for aircraft. Other pigments and particularly titanium dioxide (TiO.sub.2) including the spherical TiO.sub.2 particles and the vesiculated beads e.g. beads containing TiO.sub.2 may be included as a pigment together with the three zinc salts disclosed herein. The coating composition of this invention may be applied as one coat directly to various hard surfaces such as metal, wood, composites and does not require a topcoat to provide a high-gloss, corrosion-resistant finish.
It is generally known that low gloss coatings are appropriate for camouflage purposes particularly on most of the outer exposed surfaces of military aircraft, but low gloss coatings are not appropriate for the internal or unexposed surfaces such as the engine inlet, ducts, landing gear, etc. Nonmilitary aircraft, particularly require high gloss, high visibility coatings. Thus, it was heretofore believed that in order to obtain good corrosion resistance the pigment volume concentration (PVC) had to be relatively high thereby also resulting in a low gloss finish. It was therefore believed that it was not possible to obtain a coating which had a high gloss and at the same time good corrosion resistance.